$\left(5x - 5\right)\left(-10x + 10\right) = \ ?$
Explanation: $= 5x \cdot \left(-10x + 10\right) - 5 \cdot \left(-10x + 10\right)$ $= \left( 5x \cdot -10x \right) + \left( 5x \cdot 10 \right) + \left( -5 \cdot -10x \right) + \left( -5 \cdot 10 \right)$ $= -50x^2 + \left( 5x \cdot 10 \right) + \left( -5 \cdot -10x \right) + \left( -5 \cdot 10 \right)$ $= -50x^2 + \left( 50x + 50x \right) + \left( -5 \cdot 10 \right)$ $= -50x^2 + 100x + \left( -5 \cdot 10 \right)$ $= -50x^2 + 100x - 50$